In an internal combustion engine for an automobile, exhaust gas recirculation (EGR) for recirculating part of exhaust gas to an intake side is widely implemented. The execution of the EGR can reduce NOx in the exhaust gas, and at the same time, can improve the fuel efficiency.
The effect of the EGR can be increased by introducing more EGR gas into the cylinder, that is, by increasing the EGR rate in the cylinder. However, on the other hand, the higher the EGR rate in the cylinder, the higher the possibility of variations in the EGR rate between the cylinders and the higher the possibility of degradation in the combustion. To prevent the situation, the EGR rate in the cylinder needs to be accurately specified. It is also necessary to precisely control the EGR rate to prevent the degradation in the combustion.
However, the EGR rate in the cylinder cannot be directly measured and can only be indirectly specified based on some information. A heat release rate and a combustion period are conventionally used as the information. It is known that the EGR rate in the cylinder is closely related to the heat release rate and the combustion period. Although the heat release rate and the combustion period cannot be directly measured, an internal combustion engine including an in-cylinder pressure sensor can use an output signal of the in-cylinder pressure sensor to calculate the heat release rate and the combustion period. For example, Japanese Patent Laid-Open No. 2000-054889 describes calculation of the heat release rate at each crank angle from an output signal of an in-cylinder pressure sensor and control of the EGR rate to bring the heat release rate pattern in line with a predetermined waveform pattern.
However, since the combustion state of the internal combustion engine is affected by a formation condition of an air-fuel mixture in the cylinder or an ignition status, variations in the heat release rate and the combustion period occur between cycles even if the same operation state is maintained. Therefore, statistical processing of the variations is necessary to accurately estimate the EGR rate from the heat release rate and the combustion period, and a large number of samples are necessary. The higher the number of samples, the higher the estimation accuracy of the EGR rate. However, a large number of cycles are consumed accordingly, and the responsiveness of the EGR rate control is reduced.
Meanwhile, for example, as described in Japanese Patent Laid-Open No. 7-189815, a method of using information independent of the combustion state to specify the EGR rate is also known. In the method described in the publication, an output signal of an in-cylinder pressure sensor at an intake stroke is used as information for specifying the EGR rate. It is a known fact that the intake pipe pressure changes depending on the EGR rate, and the in-cylinder pressure and the intake pipe pressure at the intake stroke, in which an intake valve is open, are correlated. Therefore, in an internal combustion engine including the in-cylinder pressure sensor, the output signal of the in-cylinder pressure sensor at the intake stroke can be observed to indirectly specify the EGR rate in the cylinder.
The conventional method for specifying the EGR rate from the output signal of the in-cylinder pressure sensor at the intake stroke can be specifically described using a flow chart of FIG. 8 and an in-cylinder pressure/crank angle diagram of FIG. 9. The in-cylinder pressure/crank angle diagram of FIG. 9 illustrates a change in the in-cylinder pressure from an intake stroke to an exhaust stroke, wherein a case with the EGR and a case without the EGR are compared.
As shown in the flow chart of FIG. 8, in the conventional method, an output signal of an in-cylinder pressure sensor (CPS) is read (step S11), and the read output signal is multiplied by a predetermined gain to convert a voltage value to a pressure value (step S12). However, although an absolute pressure of the in-cylinder pressure is necessary to specify the EGR rate, since the in-cylinder pressure sensor outputs a change in the pressure by a voltage, the pressure value converted from the voltage value includes an offset relative to the absolute pressure of the in-cylinder pressure. Therefore, as expressed by an arrow S13 in FIG. 9, absolute pressure correction of the pressure value converted from the voltage value is performed (step S13). Examples of the method of absolute pressure correction include the following methods. In an example of the method, it is assumed that the in-cylinder pressure at the intake stroke is equal to the measurement value by the intake pipe pressure sensor, and the absolute pressure correction value is determined based on the output signal of the intake pipe pressure sensor. In another example of the method, the compression stroke following the intake stroke is assumed as adiabatic compression, and the absolute pressure correction value is determined to realize PVk=certain relationship.
After the absolute pressure correction, as expressed by an arrow S14 in FIG. 9, the in-cylinder pressure at the intake stroke is acquired (step S14). More specifically, the in-cylinder pressure after the absolute pressure correction is used to calculate an indicated average effective pressure in an interval of the intake stroke. In the conventional method, the in-cylinder pressure at the intake stroke obtained in this way is compared with the in-cylinder pressure in the case without the EGR to specify the EGR rate in the cylinder from the difference (step S15). The in-cylinder pressure at the intake stroke as information for specifying the EGR rate does not depend on the combustion state, unlike the heat release rate, the combustion period, and the like. Therefore, according to the method of using the in-cylinder pressure at the intake stroke, it is expected that the EGR rate can be specified more accurately than in the method of using the heat release rate and the combustion period.
However, in reality, it is difficult to ensure the accuracy of the method for specifying the EGR rate using the in-cylinder pressure at the intake stroke, as in the case of using the heat release rate and the combustion period. Although the absolute pressure correction is necessary to obtain the in-cylinder pressure at the intake stroke from the output signal of the in-cylinder pressure sensor, inclusion of an error in the absolute pressure correction value cannot be prevented in any of the correction methods. Moreover, since the magnitude of the error is in a similar order as the change in the in-cylinder pressure at the EGR execution, the effect of the error in the absolute pressure correction on the accuracy of specifying the EGR rate is significantly large.